In short. Parallel generators must divide real power in proportion to their ratings and reactive power in a stable way. Droop gives simple sharing, isochronous load sharing keeps frequency constant, and a power management system adds start-stop by demand, load shedding and fault response.
Sharing real power (kW)
| Method | How it works | Notes |
|---|---|---|
| Droop | Each governor reduces speed slightly with load; the units settle at shared load | Simple and robust; frequency varies with load |
| Isochronous with load-sharing lines | Controllers exchange load information and hold frequency constant | Standard for island plants |
| Digital communication | Controllers share data over a network | Flexible; depends on network health, with fallback |
| Base load / fixed load | A unit holds a set output | Used when paralleled with the utility |
Sharing reactive power (kVAr)
Voltage regulators must share reactive power. Droop compensation or cross-current compensation prevents units fighting each other. Poor sharing causes circulating currents and overheating. See kW and kVAr sharing.
Power management functions
- Load-dependent start and stop: start another unit when load exceeds a threshold, stop one when load falls.
- Spinning reserve: keep enough capacity online for the largest step or failure.
- Load shedding: drop non-essential loads if capacity is lost. See load shedding.
- Fault response: restart a failed unit or bring a standby unit in.
- Running-hours balancing across units.
Commissioning steps
- Check governor and AVR settings and droop on each unit.
- Test sharing across low, mid and high load with a load bank. See load bank testing.
- Test step loads and unit tripping.
- Simulate loss of the communication network and confirm fallback behaviour.
- Verify load-shedding priorities.
Illustrative example
A plant of three 1,000 kVA units and one 500 kVA unit shares load in proportion to rating, so the small unit takes half the load of the others. At 40 percent load, the system runs one unit, and a second starts when load passes 80 percent of the running capacity. A reserve rule keeps enough capacity to handle the largest motor start.
What load sharing must achieve
Paralleled sets should each carry a fair proportion of both real and reactive load. Real power sharing is governed by the speed governors. Reactive sharing is governed by the AVRs.
Droop: frequency falls by a set percentage (often 3 to 5) from no-load to full load, so two sets with equal droop share kW in proportion to rating
Sharing methods
| Method | Notes |
|---|---|
| Droop | Simple, no communication, but frequency varies with load |
| Isochronous with load-share line | Keeps frequency steady, needs reliable signal link |
| Digital communication based | Flexible, supports power management |
Example (illustrative)
Two 500 kVA sets feed 600 kW. With matched droop, each takes about 300 kW. If one governor has a different droop, it takes more or less, and the sharing becomes unequal. Correct by adjusting droop or using load-share control.
Power management functions
- Start the next set when demand rises above a threshold.
- Stop a set when demand falls, subject to minimum run time.
- Load shedding when capacity is short. See load shedding.
- Run-hour balancing across sets.
Common problems
- Circulating kVAr from AVR mismatch.
- Hunting from governor tuning conflicts.
- Faulty load-share line wiring.
See N+1 design.
Common mistakes
- Different droop settings between units.
- No fallback if the sharing network fails.
- Starting additional units too late for load growth.
What to send us
Send the generator list, load profile and control details. Send your application details. See also load-sharing controls.
Questions engineers ask
Why do my generators not share equally?
Common causes are mismatched droop, governor or AVR settings and measurement errors.
Is droop or isochronous better?
Droop is simple and robust. Isochronous suits island plants needing constant frequency.
What is spinning reserve?
Online capacity above the current load, held for steps and failures.
What happens if the network between controllers fails?
A good design falls back to droop or a safe mode.
This guide gives general principles. Settings need testing on the actual plant.